WASHINGTON — The Affordable Care Act requires insurers to cover mental health care just as they do physical care, but a new study shows only half of psychiatrists accept insurance. That means access to care for the millions of people with depression, post-traumatic stress disorder and other mental health issues may be limited to those who can pay for treatment out of their own pockets, despite the law.

From 2009 to 2010, 53% of psychiatrists accepted insurance, compared with 89% of all other physicians who did, said Tara Bishop, associate professor of public health and medicine at Cornell Medical College. She looked at data from the National Center for Health Statistics and released her team's findings in The Journal of the American Medical Association.

"We saw declines in the last few years in rates of acceptance, and we were wondering why," Bishop said. "I think we've all heard a lot of patient stories and doctor stories about trying to find a psychiatrist who takes insurance."

She said she was surprised by the 36-percentage-point discrepancy.

"It seemed to be getting worse in more recent years," she said. "We saw similar things for Medicare: 54.8% of psychiatrists took Medicare, as opposed to 86% of other physicians."

According to the National Institute of Mental Health, about one of five adult Americans, or about 58 million people, suffer from a diagnosable mental illness in a given year.

Insurers must change the way they cover mental health in 2014 for two reasons:

•In 2008, Congress passed the Paul Wellstone and Pete Domenici Mental Health Parity and Addiction Equity Act, which demands that mental illnesses be treated the same as other illnesses.

•In 2010, President Obama signed the Affordable Care Act, which includes mental health parity as a requirement for insurers.

But nothing in the law requires psychiatrists to take insurance, and after years of low rates, administrative hassles, extra steps not required of other physicians and a plethora of patients willing to pay cash, many psychiatrists have simply opted out of insurance programs.

Bishop speculated that two things might be at work: Psychiatrists may feel they don't get reimbursed by insurers at rates that are worth their time. And fewer medical students are studying psychiatry, which could create a higher demand for those who do.

"You need somewhat of a market in order not to take insurance," she said. "So it may be there's enough of a demand and not enough of a supply."

Add to that the administrative burden of dealing with insurers, and there isn't much reason for a psychiatrist working from a small office to take insurance, she said.

Together, that means that those who don't have the cash to pay out $100 to $200 for a session may be left without recourse.

"I think the parity laws are very important to ensure that people have the same coverage for mental health," Bishop said, "but they don't address this kind of issue. You may require insurers to pay for that care, but you may not have the providers to provide that care."

There are safety-net options, such as outpatient clinics and community health centers, Bishop said. She worries that too many people end up at the emergency room with exacerbated mental health issues that could have been prevented by access to a psychiatrist. She hopes to conduct similar research about how many psychologists and social workers, such as marriage counselors, take insurance.

"I think it's going to take multiple stakeholders and minds to address the crisis of access to mental health care in this country," she said.

Possibilities might include looking toward how the primary care community has tried to address a projected doctor shortage, such as by working as teams; changing the reimbursement system; encouraging students to go to medical school; giving doctors incentives to take insurance; and training others in the medical field, such as nurse practitioners, to take on some of the doctors' workload.

This month, Vice President Biden announced a $100 million initiative to "increase access to mental health services and improve mental health facilities." Half the money comes through the Affordable Care Act to help community centers provide more services, and half will go to the Department of Agriculture to help finance rural mental health facilities. That, Bishop said, should help.

Jamison Monroe of Newport Academy, a mental health and addiction facility for people ages 12 to 20 in Connecticut and California, said he wasn't surprised by Bishop's findings. The Connecticut facility opened two months ago because doctors on the East Coast said they had nowhere to send their young patients. It's already full.

About half of his patients pay through insurance, and the other half do not. Recently, he said, his insurance manager got authorization from an insurer for treatment for one of his patients, then refused to pay.

"There's been some apprehension about providing for those with insurance because we're left holding the bag," he said.

Small providers probably won't take an insurer to court, he said. "Sole practitioners don't have any recourse. That's why they only take cash."

Linda Rosenberg, president of the National Council for Behavioral Health, called mental health parity, the provisions in the new health law and the shortage of providers an "interesting kind of perfect storm."

It may spark better understanding of mental health and further breakdown of the stigma that often surrounds brain-chemistry diseases such as depression, schizophrenia or post-traumatic stress disorder.

It could also make the treatment process easier for patients. "Even when people had insurance coverage, there were steps that were much more onerous than that in other medical fields," she said, such as having to "requalify" for treatment with permission from the insurer after 10 visits, or having to go through a subcontractor who handled mental health, or having to buy a separate policy just for mental health. Now their cases will be handled the same as other patients.

"That's the good-news story," she said.

But psychiatrists are used to being paid outside the insurance world.

Therapists have dealt with low rates from insurers, extra steps in receiving payment and high demand for treatment by simply taking themselves out of the administrative hassle, she said. Provisions in the ACA require that psychiatrists be paid at rates similar to other doctors, though no one knows exactly what that will look like. Until the rates go up, doctors will continue to choose specialties that pay better or to take cash over insurance, she said. Bringing more psychiatrists into the pool and ironing out rate discrepancies will take some time.

"When the rates go up, you'll see more people attracted to psychiatry," and that will cause the market to even out, Rosenberg said. "But rates have to go up, quite frankly. It's going to be a rough patch."